This invention relates to the construction of an anodizing film which improves the corrosion resistance of an aluminum alloy product or part used in the presence of water, such as a boat propeller or hull used in a sea or lake, a water pump or sprayer driven by a general-purpose engine or an agricultural machine used in a paddy field.
Every such product or part is usually covered with a rust- or corrosion-preventing surface coating. Of particular importance is a rust-proofing coating which enables any such product or part to stand seawater containing salt as an element accelerating corrosion. A large number of technical proposals have hither to been made about rust- or corrosion-preventing coating, and include Japanese Patent Laid-Open Publication No. HEI-2-250997 entitled xe2x80x9cMethod for Rust-Proofing Treatment of Aluminum Material and an Aluminum Outboard Enginexe2x80x9d, which is characterized by forming an anodizing film on the surface of an aluminum or aluminum alloy material, sealing its pores with molybdenum disulfide and forming a surface coating thereon.
The proposed aluminum outboard engine has, however, been found to exhibit only the corrosion resistance which was satisfactory as intended in those days, but which never satisfies the severe requirements of recent years.
This invention provides a rust-proofing structure of improved corrosion resistance.
We, the inventors of this invention, have paid attention to the proposed use of molybdenum disulfide and found a problem as a result of our study. More specifically, we have reached the conclusion that molybdenum disulfide is a crystalline substance which does not form any layer of passivity with aluminum, but merely closes the pores of an anodizing film and adheres to it, and is not satisfactory in durability. We have, therefore, studied the use of an amorphous substance capable of forming a layer of passivity with aluminum and succeeded in establishing technology which can overcome the problem of the prior art.
According to a first aspect of this invention, there is provided a process for improving an anodizing film which comprises the steps of forming an anodizing film on the surface of an aluminum alloy material, impregnating the film with an amorphous substance capable of forming a layer of passivity with aluminum to fill pores formed in the film during its growth, and sealing the pores to close their inlets to confine the amorphous substance therein.
The anodizing film is impregnated with an amorphous substance so that a layer of passivity may be formed between the amorphous substance and aluminum if the film should have any flaw reaching the aluminum alloy material. The formation of such a layer makes it possible to restrain corrosion.
According to a preferred example of this invention, zirconium phosphate is used as the amorphous substance. Zirconium phosphate or chromium chromate can be used as an amorphous substance forming a layer of passivity with aluminum, but chromium chromate is a harmful heavy metal and gives waste water incurring a high cost of disposal. Zirconium phosphate is preferred, since it does not require any such costly treatment, but enables a reduction of cost for an improved anodizing film. According to a preferred example, it is adequate to use zirconium phosphate in the amount of about 15 mg/cm2 to 45 mg/cm2.
Even boiling water or sodium silicate can be used for the sealing of the pores. The use of nickel acetate is, however, preferred for corrosion resistance to boiling water or sodium silicate.
The process may further include the step of coating the surface of the anodizing film with a primer after the sealing of its pores. The anodizing film and primer protect the aluminum alloy material if they are sound, while if the film is damaged, zirconium phosphate and aluminum form a layer of passivity which protects the aluminum alloy material. According to a preferred example, phosphomolybdic acid is used as a pigment for the primer. Zinc phosphate or tripolyphosphoric acid can also be used as the pigment. Phosphomolybdic acid is, however, preferred for corrosion resistance. According to a preferred example, it is adequate to use phosphomolybdic acid in the proportion of 5 to 15% by weight.
According to a second aspect of this invention, there is provided an anodizing film structure which comprises an anodizing film formed on the surface of an aluminum alloy material, zirconium phosphate impregnating the film to fill pores formed therein during its growth, and a seal closing the inlets of the pores.
According to the structure, the anodizing film is impregnated with an amorphous substance so that a layer of passivity may be formed between zirconium phosphate and aluminum if the film should have any flaw reaching the aluminum alloy material. The formation of such a layer makes it possible to restrain corrosion. The film having its pores sealed protects the aluminum alloy material if it is sound, while if it is damaged, zirconium phosphate and aluminum form a layer of passivity which protects the aluminum alloy material. The structure, therefore, provides a long time of protection for the aluminum alloy material. As regards the amount of zirconium phosphate as the impregnant, it is adequate to use about 15 mg/cm2.
The structure may further include a primer layer formed on the surface of the anodizing film including the seal. The primer layer and the film having its pores sealed protect the aluminum alloy material if they are sound, while if the film is damaged, zirconium phosphate and aluminum form a layer of passivity which protects the aluminum alloy material. The structure, therefore, provides a still longer time of protection for the aluminum alloy material. The primer layer may be of a primer containing phosphomolybdic acid as a pigment. Zinc phosphate or tripoly-phosphoric acid can also be used as the pigment. Phosphomolybdic acid is, however, preferred for corrosion resistance.
According to a third aspect of this invention, there is provided an aluminum-alloy-made outboard engine which comprises an engine cover, an under cover attached to the bottom of the engine cover, an extension case attached to the bottom of the under cover and a gear case attached to the bottom of the extension case, each of at least the extension and gear cases having an anodizing film structure comprising an anodizing film formed on the surface of an aluminum alloy material, zirconium phosphate impregnating the film to fill pores formed therein during its growth, and a seal closing the inlets of the pores.
The outboard engine is exposed to salt water and sea breezes, and the gear and extension cases are, among others, exposed to seawater and attacked by chlorine ions in seawater, etc. The anodizing film covers the aluminum alloy material to prevent its corrosion by chlorine ions, and the pores of the film are filled with zirconium phosphate and closed at their inlets. The anodizing film structure gives the outboard engine a greatly improved corrosion resistance and protects it from corrosion.